1. Field of the Invention
This invention relates generally to methods for producing integrated circuits, and more particularly to methods for compensating for the E-beam proximity effect.
2. Description of the Prior Art
An integrated circuit comprising many hundreds or thousands of individual components is typically formed on a semiconductor substrate, such as a wafer of silicon. To produce integrated circuits, patterned layers of semiconducting, insulating, and conducting materials are sequentially provided over the surface of the substrate until the desired circuits are completed.
A typical method for patterning a layer of an integrated circuit is to apply a reactive material to an upper surface of the substrate, expose the reactive material to a source of radiant energy, and to develop the reactive material to produce a "mask" over the surface of the substrate. The layer is then patterned through the mask and the mask is subsequently removed. The process for forming the mask is often referred to as lithography.
Most often, the radiant energy to which the reactive material is exposed is electromagnetic radiation in the visible or ultraviolet range, the reactive material is photoresist, and the process is referred to as photolithography. However, as the size of integrated circuits decrease the relatively long wavelengths of visible and ultraviolet light become a limiting factor in the possible feature resolution of the mask. In consequence, there is considerable interest in utilizing radiant energy of shorter wavelengths to expose the reactive layers. One type of radiant energy which shows great promise in lithography comprises a beam of electrons, or E-beam.
While the E-beam lithography process is capable of superior feature resolution, the process also has its share of problems and disadvantages. One problem of major concern with E-beam lithography is the so-called "proximity effect".
Briefly, the E-beam proximity effect is caused by the back-scattering of electrons as the E-beam impacts the reactive layer, underlying layers, and the substrate. The back-scattering increases the effective exposure of portions of the reactive layer, causing undesirable variations in feature sizes. While the E-beam proximity effect is not particularly troublesome in widely separated features, it can cause considerable variation in the sizes of closely-packed features, such as those found in grating patterns.
There have been many proposed correction techniques for the E-beam proximity effect. One such method is described by Owen, et al., in "Proximity Effect Correction for Electron Beam Lithography by Equalization of Background Dose", Journal of Applied Physics, Vol. 54, No. 6, June, 1983. The Owen, et al. correction method, which is named "Ghost", involves first exposing the image area in a traditional manner, and then exposing the field area with a de-focused beam at a lower dose to compensate for the back-scattered electron energy. In consequence, all of the image areas receive the same level of energy, and the background energies in the field are equalized. However, the ghost method requires an additional field exposure, and results in lower image contrast.
The proximity effect can be compensated for by several other methods, including the adjustment of the exposure dose for each subdivided shape, and the adjustment of the size of the design pattern. Parkah, in the Journal of Applied Physics, Vol. 50, No. 6, Page 4371 (1979), teaches the former method; and Wittels, et al., Electron and Ion Beam Science and Technology, Electrochemical Society (1978), teaches the latter method. Unfortunately, both methods require massive computer and manual calculations of such factors as local exposure doses, and the size, shapes, and geometries of adjoining features and spaces. These calculations tend to be very expensive and time consuming.